Energy absorbent laminate

ABSTRACT

This invention provides multi-layered composites, laminates and composite joints in which at least one resin-impregnated, fiber-containing layer is joined or laminated to a core layer having a lower flexural modulus or higher elongation at break, higher toughness, or a combination of all or some of these properties. The multi-layer composite produced by laminating or joining these materials together has improved shearout, impact and cutting resistance, since stresses caused by outside forces can be more widely distributed throughout the composite.

FIELD OF THE INVENTION

This invention relates to composite joints generally, and morespecifically, to composite joints including at least one multi-layeredcomposite having at least two layers of different toughness for helpingto retard bearing stress and shearout stress.

BACKGROUND OF THE INVENTION

Fiber-reinforced composites are relatively brittle compared toconventional ductile metal alloys, such as stainless steel and aluminum.Yielding of ductile metals usually reduces the stress concentrationaround bolt holes so that there is only a loss of area, with no stressconcentration at ultimate load on the remaining section at the joints.With composites, however, there is no relief at all from the elasticstress concentration, and catastrophic failure usually results withoutmuch warning. Even for small defects in composite structures, thestress-concentration relief is far from complete, although the localdisbanding between the fibers and resin matrix and local intraply andinterply splitting close to the hole edge does locally alleviate themost severe stress concentrations. Since the stress resistant capabilityof bolted and riveted joints in composite materials is oftenunacceptably low, such laminates can never be loaded to levels suggestedby the ultimate tensile strength of the laminated composite itself.

It is recognized that the strength of a composite structure with bothloaded and unloaded holes depends only slightly on the fiber pattern.Indeed, throughout the range of fiber patterns surrounding laminatedstructures, the bearing strength and gross-section strengths are almostconstant, which simplifies the design process.

The design and analysis of bolted or riveted joints in fibrouscomposites remains very much an art because of the need to rely onempirical correction factors in some form or another. Mechanicallyfastened joints differ from bonded composite joints because the-presenceof holes insures that the joint strength never exceeds the locallaminate strength. Indeed, after years of research and development, itappears that only the most carefully designed bolted composite jointswill be even half as strong as the basic laminate. Simpler bolted jointconfigurations will typically attain no more than about a third of thelaminate strength. However, because thick composite laminates are oftenimpossible or impractical to adhesively bond or repair, there is acontinued need for bolted composite structures.

Since bolted composite structural joints are so brittle, it is veryimportant to calculate accurately the load sharing between fasteners andto identify the most critically loaded one. Bolted joints of compositematerials are known to experience many modes of failure, includingtension failure, shearout failure, bolts pulling through laminatefailure, cleavage tension failure, bearing failure, cutting, impact andbolt failure. See COMPOSITES, Engineered Materials Handbook V1.1, pp.479-495 (1987), which is hereby incorporated by reference.

The use of local softening strips and pad-ups, has been known toalleviate some of the stress concentrations with respect to basiclaminate structures. However, such an approach is not without drawbacks,since these modifications leave the structure outside the locallyprotected areas with little, if any, damage tolerance because the higheroperating strain permitted by the softening strips and pad-ups severelylimits the opportunity to perform repairs, which limits the number ofsituations in which such an approach is practical.

Accordingly, there remains a need for improving the failure resistenceof composite structures. In addition, laminate composite technologyneeds to improve upon the existing design structures to minimizefailures associated with shear, bearing, cutting and impact forces.

SUMMARY OF THE INVENTION

Multi-layered composites useful in high loading applications areprovided by this invention. In a first preferred embodiment of amechanically fastened composite joint of this invention, a substrate anda multi-layered composite are provided. The composite includes a pair ofresin-impregnated, fiber-containing layers. The composite furtherincludes a fiber-containing core layer having a lower tensile modulus,higher toughness, and/or higher elongation at break than the resinimpregnated, fiber-containing layers. The core layer is sandwichedbetween the pair of resin-impregnated, fiber-containing layers. Uponsubjecting this composite to high external forces, the resultingcomposite joint has improved shearout, cutting and impact resistanceover that which would be expected if the composite layer were made inthe same thickness without a core layer.

The multi-layered composites and laminates of this invention exhibitgood tensile and flexural strength and moduli due to the strong tensilemodulus layer, while surprisingly, also exhibit excellent bearingshearout, cutting and impact resistence due to the second layer or corelayer having greater energy absorbing properties.

During shearout testing, a hole is drilled near the edge of themulti-layered composite, and a bolt is inserted. The force at failurecaused by pulling the bolt in the direction of the plane of thecomposite is measured. This force is the shearout resistence of thecomposite to tiering or plowing. The multi-layered laminates of thisinvention have significantly higher shearout resistence than compositesof similar thickness made from consolidated plies having the same resinand reinforcement dispersed throughout. While not being committed to anyparticular theory, it is believed that the lower integrity, tougher ormore ductile second or core layer spreads the load, for example, to thesides of the hole and beyond the typical bearing and tangential (hoop)stress areas, such that several inches of the composite may becomeinvolved in stress relief. The preferred fibers in the core layer canabsorb high amounts of energy, such as by elongating in a ductilefashion, delaminating from the skins, or bunching during delamination,to act in concert to resist damaging forces due to cutting, impact orshear.

In a further preferred embodiment of this invention, an energy absorbentlaminate is provided. This laminate includes a pair ofresin-impregnated, fiber-containing layers and a fiber-containing corelayer having a higher toughness and greater elongation at break thansaid resin-impregnated, fiber-containing layers. The core layer issandwiched between the pair of resin-impregnated, fiber-containinglayers to form an integral composite. The integral composite hasimproved shearout, cutting and impact resistence over a composite ofapproximately the same thickness made without the core layer.

Further improvements offered by this invention are the use of core orsecond layers composed of lower modulus, higher elongation fibers,poorly wetted or weakly bonded high modulus fibers, in the form of yarn,roving, tow, woven fabric, non-woven fabric, or combinations thereof.The controlled, limited adhesion may be achieved by using the same, ordifferent, resin matrix as in the first or outer layers, or by joiningonly some of the individual fibers in the core layer together by meltingor curing. Alternatively, the core can contain no matrix resin at all,so that it readily absorbs external forces. If translucency is required,a low strength additive, such as polypropylene copolymer wax may be usedto substantially eliminate air voids in the laminate structure.

In further developments of this invention, the composite or laminatestructure can include essentially only two materials, such aspolypropylene resin and glass, to increase recyclability. Recyclabilityis known to be improved by reducing the number of materials which can beseparated from the composite.

Finally, this invention can include a multi-layered laminate including afiber reinforced pair of outer skins and a core layer including anaramid fiber reinforcement having greater toughness. This high strengthcomposite is particularly suitable for cockpit doors,explosion-resistence panels, such as air cargo containers, andbullet-proof vests.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of theinvention, in which:

FIG. 1: is a side elevational, perspective view of a multi-layeredcomposite of this invention illustrating fiber reinforcement in phantom;

FIG. 2: is a side elevational perspective view of a single ply examplefor use with the multi-layered composite of this invention;

FIG. 3: is a diagrammatical perspective view of a bolted joint compositeof this invention undergoing shear-out failure; and

FIG. 4: is a substrate-supported multi-layered composite of thisinvention showing rivet and bolt fasteners.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Multi-layered composites and laminates are provided by this inventionwhich have greater resistence to cutting, impact, bearing, hoop, andshearout forces when used alone or in connection with mechanicalfasteners. The composite materials of this invention can be used forautomobile and aircraft body panels, highway and road signs, truckpanels, such as hoods and fenders, seats and panels for transit cars,boat hulls, bathroom shower-tub structures, chairs, architecturalpanels, agricultural seed and fertilizer hoppers, tanks and housings fora variety of consumer and industrial products. Further applicationsinclude printed-circuit boards, gears and sporting goods, such as skis,ski boards, and fishing poles. Aramid fiber embodiments of thisinvention can be useful in military structures and bullet-proof vests,as well as explosion-resistant panels for air cargo containers andcockpit doors. As used herein, the following terms are defined:

“Composite”—means any combination of two or more materials (such asreinforcing elements, fillers, etc., and a composite matrix binder)differing in form or composition on a macro scale. The constituentsretain their identities: that is, they do not dissolve or mergecompletely into one another although they act in concert. Normally, thecomponents can be physically identified and interface between oneanother.

“Laminate”—means a product made by uniting laminae or plies via bondingthem together, usually with heat, pressure and/or adhesive. Whilenormally referring to flat sheet, laminates can also include rods andtubes, and other non-planar structures.

“Fabric”—means a cloth which can be, for example, non-woven, needled,woven, knit or braided fibrous material, such as yarn, tow, roving orindividual fibers.

“Mat”—means a fibrous material consisting of randomly oriented choppedfilaments, short fibers, or swirled filaments loosely held together witha binder.

“Roving”—means a number of yarns, strands, tows, or ends, collected intoa parallel bundle with little or no twist.

“Tensile Modulus” (also Young's modulus)—means the ratio of normalstress to corresponding strain for tensile or compressive stresses lessthan the proportional limit of the material.

“Tensile Strength”—means the maximum load or force per unitcross-sectional area, within the gauge length, of a specimen. Thepulling stress required to break a given specimen. (See, for example,ASTM D579 and D3039, which are hereby incorporated by reference).

“Elongation”—means deformation caused by stretching. The fractionalincrease in length of a material stressed and tensioned (when expressedas a percentage of the original gauge length, it is called percentageelongation.)

“Elongation At Break”—means elongation recorded at the moment of ruptureof the specimen, often expressed as a percentage of the original length.

“Basis Weight”—means the weight of a fibrous material, such as a fabric,mat, tape, etc., per unit area (width x length). Also sometimes calledthe “Areal Weight”.

“Recyclability”—means the propensity of a material to be reused,reprocessed or remelted into the same or different product.

“Toughness”—means the amount of work required to cause failure,expressed as the area under the stress-strain curve of a test material.The absence of brittleness.

With reference to the Figures, and more particularly to FIGS. 1 and 2thereof, there is shown a preferred multi-layered composite 100 having apair of resin-impregnated, fiber-containing layers 10 and 30, and afiber-containing core layer 20 having a lower flexural modulus, highertoughness, higher elongation at break or combination thereof. To providea meaningful difference in properties, the core layer itself, and lessdesirably, just the individual fibers, should have approximately atleast 10%, preferably at least 30%, and more preferably, at least 50%greater toughness and/or elongation at break, or at least 10%,preferably at least 30%, and more preferably, at least 50% lower tensilemodulus. While these measured properties, as defined herein, representdistinctly different properties of a fiber or composite material, theyall relate to core's 20 ability to absorb energy from externally appliedforces.

The core layer 20 is preferably sandwiched between the pair ofresin-impregnated, fiber-containing layers 10 and 30 to form an integralcomposite, or layered with a resin-impregnated, fiber-containing layerto form a two-layered structure. In the preferred multi-layeredcomposite, the layers 10, 20 and 30 are plies of a laminatedconstruction, which can contain glass, thermoplastic and/orthermosetting materials in the form of particles, fibers or matrices.Alternatively, the layers 10, 20 and 30 could be prepared with layers ofB-stage thermosetting composites which are laid up and cured together.Additionally, the layers 10, 20 and 30 could be molded together, such asby suspending the core layer 20 in an injection mold, and molding layers10 and 30 around the core layer 20. Each of the layers 10, 20 and 30, inthis embodiment, preferably include some type of fiber, such as orientedfiber, tow, roving, and yarn, woven or non-woven fabric, web, or scrim.It is expected that some, or all of the fiber, or resin matrix may beeliminated from some of these layers, and/or subsequent layers,depending upon the end use for the laminate. For example, layers 10 and30 could be consolidated composite layers and the core 20 could containconsolidated, resin coated, matrix encapsulated, loose, bonded, ororiented fibers.

As described in FIG. 2, the nomenclature of multi-layered compositesincludes the planar directions of X and Y, as well as the verticaldirection, Z. It is known that most laminates are anisotropic, in thatthey provide different mechanical properties in the longitudinal andtransverse directions. Although woven fabric may minimize the differencein properties in the transverse “X” and longitudinal “Y” directions, thelayered interfaces between the layers 10, 20 and 30 create performancedifferences in the “Z” direction, such as intraply and interplysplitting due to impact or cutting loads. There are also design concernsrelating to the shear forces created by mechanical fasteners, such asbolts and rivets that are addressed below.

A shear test sample is described diagrammatically in FIG. 3. In such asample, a multi-layered composite 100 receives a drilled hole 101 intowhich a mechanical fastener such as a bolt or rivet is inserted. Thebolt or rivet is uniformly pulled in a single direction (along the arrowto the left) to test bearing and shearout strength. A typical shearoutfailure is illustrated by the dislodged portion 103 of the topresin-impregnated fiber-containing layer 10.

As shown in FIG. 4, a composite joint 200 having greater resistence tocutting, shear and impact forces is provided, including a substratematerial 50, multi-layered composite 100 and one or more fasteners, suchas a rivet 60 or bolt 62. Such a joint design is typical of thoseassociated with motor vehicle and aircraft body panels, and permits thereplacement of damage panels in service. It is known that the composite100 is often subjected to shearing forces, such as when a motor vehicletravels along a bumpy highway, or an aircraft exhibits pressurizationand depressurization during take offs and landings. The stresses withinthe composite joint can be caused by an expansion and contraction of thesubstrate 50, which is typically steel or aluminum, in relation to thecomposite 100, which may, or may not, expand or contract to the samedegree. Small differences of 1-10% in the thermal expansion coefficientbetween the composite 100 and the substrate 50 could have a dramaticimpact on the bearing stress at the site of the rivet 60 or bolt 62.Such composites can also be subject to cutting forces during an accidentor when cut by metal shears during a break-in, as well as impact forces,such as when impacted by a ballistic projectile or explosion gases ordebris. Composite 100 has improved resistance to failure by suchmechanisms created by using, for example, tougher, more ductile, orweakly bonded layers to absorb energy during shear. For example, thefibrous reinforcement 15 and 35 could be a fiberglass woven or non-wovenfabric having a basis weight of at least about 400 g/m², preferablyabout 500-700 g/m², and the fibrous reinforcement 25 can be a nylon,rayon, polyester, acrylic, or a polyolefin, such as polyethylene,polypropylene, or high tenacity polypropylene, for example. Suchpolymeric materials of the core layer 20 can be provided in a yarn, mat,scrim, tows, roving, woven, non-woven or knitted fabric, having a basisweight of at least about 200 g/m², and preferably about 300-500 g/m². Insuch an example, the glass fiber clearly would have a higher tensilemodulus than polyethylene or polypropylene fibers. If polypropylene isselected for the matrices 16 and 36, as well as fibrous reinforcement25, the composite could also be economically recycled, since it wouldcontain essentially only two readily heat separable materials (95 wt. %or better), e.g., glass and polypropylene.

A suitable composite material for the fiber or fibrous reinforcement 15,25 and/or 35 is available from Vetrotex International of 767 quai desAllobroges—BP 929, 73009 Chambery Cedex, France (a subsidiary to St.Gobain) under the registered trade name Twintex®. Twintex® is, forinstance, available as wound rovings, or woven fabrics, or towscomprising homogeneously intermingled long filaments of thermoplasticssuch as polypropylene, polyethylene, polyethyleneterephthlate (PET) andpolybutylterephthlate (PBT) with E-glass, the glass fiber contenttypically being 45 to 75 wt. % (20 to 50 vol %). The Twintex®manufacturing process enables the thermoplastic and glass fiberfilaments to be mixed “dry” with a high degree of control over thedistribution of the two filamentary fibers. The dry fibers could then befilled with resin, partially or fully consolidated under heat and/orpressure, or left dry as a core layer 20, and bonded to layers 10 and30, for example. Alternatively, a multi-layered composite could bemanufactured entirely from Twintex® material, by fully consolidating twoTwintex® layers for the resin-impregnated, fiber-containing layers 10and 30, and partially consolidating or loosely heat bonding a Twintex®material for the core layer 20.

Alternatively, for use in bullet or explosion proof panels and vests,the fiber reinforcement 25 in core layer 20 could be an aramid fiberreinforcement, such as Kevlar® woven or knit fabric, with or without aresinous matrix, having a basis weight of at least about 1,000-5,000g/m², while using a glass fabric of a basis weight of only about 200-600g/m² for fiber reinforcements 15 and 35. The resulting structure wouldbe stronger at its core than composite 100, since Kevlar® fiberstypically have a higher tensile or Young's modulus than glass fibers,but would still absorb ballistic forces, since Kevlar® fibers typicallyhave greater toughness and elongation at break than glass fibers.

The polymer resins compositions 16, 26 and 36 of composite 100 could bethe same resin, so as to improve recyclability, or different resins, toenable, for example, better binding to themselves or to differentreinforcement selections for fibrous reinforcements 15, 25 and 35. Inone preferred embodiment of this invention, a single fiber compositionis used for the fibrous reinforcements 15, 25 and 35, with a lower basisweight fabric selected for the core layer reinforcement 25 than theouter layer reinforcements 15 and 35. Additionally, most preferredembodiments of this invention also include the same resin employed forthe resin compositions 16, 26 and 36, or the elimination of resin 26entirely, so that the final composite 100 can be more easily recycled.Conventional recycling of composite materials typically enables twophase systems, such as glass fiber and a single thermoplastic resin tobe readily separated, for example, by melting the resin above theresin's melting point, but below the melting point of glass.

In accordance with the preferred embodiments of this invention, thefollowing material selection information is provided.

Fibers used in the multi-layer composite 100-embodiment of thisinvention can be selected from tough, lower modulus resinous or naturalfibers and high-strength, textile-type fibers, the latter of which aretypically coated with a binder and coupling agent to improvecompatibility with the resin, and a lubricant, to minimize abrasionbetween filaments. The fiber-resign matrix compositions 16, 26, and 36for layers 10, 20 and 30 can be supplied as ready-to-mold compounds suchas sheet molding compounds (“SMC”) or bulk molding compounds (“BMC”).These layers 10, 20 and 30 may contain as little as 5 wt. %, and as muchas 80 wt .% fiber by weight. Pultruded shapes (usually using a polyestermatrix) sometimes have higher fiber contents. Most molded layers, forbest cost/performance ratios, contain about 20 to 60 wt. % fiber.

Practically all thermoplastic and thermoset resins useful herein asmatrices and/or fibers are available in fiber-reinforced compounds,prepregs, lay-ups, and rolls. Those suitable for this invention includeepoxy, phenolics, polyester, melamine, silicone and/or polyamidethermosetting compositions, and nylon, polypropylene, polyethylene,unsaturated polyester, polyvinylchoride, polystyrene, ABS, and/or SANthermoplastics. The higher performance thermoplastic resins—PES, PEI,PPS, PEEK, PEK, and liquid-crystal polymers for example—are suitable inthe reinforced layers of this invention.

Fiber reinforcement improves most mechanical properties of plastics by afactor of two or more. The tensile strength of nylon, for example, canbe increased from about 10,000 psi to over 30,000 psi, and thedeflection temperature to almost 500° F., from 170° F. A 40 wt. %glass-fortified acetyl has a flexural modulus of 1.8×10⁶ psi (up fromabout 0.4×10⁶), a tensile strength of 21,500 psi (up from 8,800), and adeflection temperature of 335° F. (up from 230° F.). Reinforcedpolyester has double the tensile and impact strength and four times theflexural modulus of the unreinforced resin. Also improved in reinforcedcompounds are tensile modulus, dimensional stability, hydrolyticstability, and fatigue endurance.

The multi-layered composite 100 can also be a laminate. Laminatedplastics are a special form of polymer-matrix composite, which oftencontain layers of reinforcing materials that have been impregnated withthermosetting or thermoplastic resins, bonded together, and cured orformed under heat and pressure. The cured or formed laminates, calledhigh-pressure laminates, can be provided in more than 70 standardgrades, based on National Electrical Manufacturers Association (NEMA)specifications, which are-hereby incorporated by reference.

Laminated plastics are available in sheet, tube, and rod shapes that arecut and/or machined for various end uses. The same base materials arealso used in molded-laminated and molded-macerated parts. Themolded-laminated method is used to produce shapes that would beuneconomical to machine from flat laminates, where production quantitiesare sufficient to warrant mold costs. The strength of a molded shape ishigher than that of a machined shape because the reinforcing plies arenot cut, as they are in a machined part. The molded-macerated method canbe used for similar parts that require uniform strength properties inall directions.

Other common forms of laminated plastics useful for composite 100 arecomposite sheet laminates that incorporate a third material bonded toone or both surfaces of the laminate. Metals most often used incomposites are copper, aluminum, nickel, and steel. Nonmetallics includeelastomers, vulcanized fiber, and cork.

Vulcanized fiber is another product often classified with the laminatedplastics because end uses are similar. Vulcanized fiber is made fromregenerated cotton cellulose and paper, processed to form a densematerial (usually in sheet form) that retains the fibrous structure. Thematerial is tough and has good resistance to abrasion, flame, andimpact.

Glass is the most widely used reinforcing material in compositesgenerally, and is a preferred fiber for fibrous reinforcements 15, 35,and less so for fibers 25. Glass fiber has a tensile strength of about500,000 psi (virgin fiber at 70° F.). All forms of glass fibers areproduced in the standard C-glass, S-glass, A-glass, ECR-glass andE-glass reinforcement types. S-glass has a tensile strength aboutone-third higher than that of E-glass, but the cost of S-glass isconsiderably higher. S-2 Glass, a product of Owens-Corning, is a variantof S-glass, having the same batch composition but without the rigid,military quality-control specifications. Properties are similar to thoseof S-glass; and the cost is between that of E and S-glass. Otherreinforcements which can be used are carbon, graphite, boron, and aramid(Kevlar®) for high-performance requirements; glass spheres and flakes,fillers such as powderized TiO₂, MgO and Al₂O₃; and fibers of cotton,jute, and synthetic materials such as olefins, for example,polyethylene, polypropylene, and polystyrene, as well as, nylon andpolyester (such as Compet and Spectra fibers available fromAllied-Signal Corp.), and ceramic materials.

Fibers are available in several forms: roving (continuous strand), tow,yarn, knits, chopped strand, woven fabrics, continuous-strand mat,chopped-strand mat, and milled fibers (hammer milled through screenswith openings ranging from 1/32 to ¼ in.). The longer fibers provide thegreater strength; and continuous fibers are the strongest.

Fibers in the composite100 can be long and continuous, or short andfragmented, and they can be directionally or randomly oriented. Ingeneral, short fibers cost the least, and fabrication costs are lower,but the properties of resulting composites are lower than thoseobtainable with longer or continuous fibers.

Other reinforcements useful in this invention include paper, cotton,asbestos, glass, and polymeric fabric, mat and scrim. Papers are thelowest-cost reinforcing materials used in making laminates. Typesinclude kraft, alpha, cotton linter, and combinations of these. Papersprovide excellent electrical properties, good dimensional stability,moderate strength, and uniform appearance. Cotton cloth also is used forapplications requiring good mechanical strength. The lighter-weightfabrics are not as strong but have excellent machinability. Asbestos, inthe form of paper, mat, or woven fabric provides excellent resistance toheat, flame, chemicals, and wear. Glass-fiber reinforcements, in wovenfabric or mat, form the strongest laminates. These laminates also havelow moisture absorption and excellent heat resistance and electricalproperties Nylon fabrics provide excellent electrical and mechanicalproperties and chemical resistance, but laminates reinforced with thesematerials may lack dimensional stability at elevated temperatures. Otherfabrics, which are especially useful for the fibrous reinforcement 25 ofthe core layer 20 include, polyolefins, such as polyethylene orpolypropylene knit, woven, non-woven fabric or scrim, or Twintex®polyolefin and glass fiber mixtures. Additionally, aramid fabrics, wovenor non-woven, could be used in ballistic applications.

Typical mechanical properties for high and low strength fibers areprovided in Table 1 and Table 2 below: TABLE 1 Core candidate fibershaving high toughness and low modulus polyester .35-.55 g-cm-% nylon 6/6.8-1.25 g-cm-% polypropylene .75-3 g-cm-% polyethylene .75-4 g-cm-%

TABLE 2 Properties of certain high strength fiber materials LongitudinalDensity, Young's modulus Tensile strength Material g/cm³ GPa 10⁶ psi MPaksi Polyester 1.36 13.8 2.0 1100 160 E-glass 2.52 72.3 10.5 3450 500S-glass 2.49 85.4 12.4 4130 600 Kevlar 49 1.44 124 18.0 2760 400 T-3001.72 218 31.6 2240 325 VSB-32 1.99 379 55.0 1210 175 FP 3.96 379 55.01380 200 Boron 2.35 455 66.0 2070 300 Silicon Carbide 3.19 483 70.0 1520220 GY-70 1.97 531 77.0 1720 250

This invention will be further described in connection with thefollowing examples:

EXAMPLE A

A tri-layered laminate was prepared using two plies of 600 g/m²consolidated polypropylene glass Twintex® fabric sheet as the outerlayers and a 400 g/m² woven polypropylene fabric as the core layer. Thecore layer was bonded to the outer layers in a controlled way using apair of polyethylene adhesive webs, such that a moderate amount ofadhesion was achieved upon heating and pressing the combination oflayers together. The resulting laminate exhibited higher shearoutresistance, high impact and flexural strength and was capable of beingrecycled due to the presence of principally one thermoplastic matrix,and one fiber type, glass. This design was highly suitable for truckroof panels, highway signs and other fastened plate uses.

EXAMPLE B

Another tri-layered laminate was prepared by laminating together two 400g/m² consolidated polypropylene-glass Twintex® sheets as the outerskins. These skins were combined with a core layer of 300 g/m² wovenaramid fiber (Kevlar®) and consolidated at 200° C. (400° F.), below themelting point of Kevlar®. Alternatively, the Twintex® sheets can beconsolidated independently and then glued or joined with polyethyleneadhesive webs, or the layers laminated with heated press rolls. TheKevlar® fiber could contain a compatible coating, such as polypropylene,to improve adhesion. The resulting composite was combined with heat andpressure, and resulted in a highly explosion-resistant panel suitablefor air cargo containers and cockpit doors.

In view of the foregoing, it can be realized that this inventionprovides improved multi-layered composite structures suitable forcomposite joints involving metallic substrates and mechanical fasteners.The preferred embodiments of this invention use a core or second layerhaving a lower flexural modulus, higher toughness and/or higherelongation at break than the skin layers or first layer for allowingbetter distribution of bearing forces due to mechanical fastenerloading. Certain other embodiments of this invention employ a singlematrix resin and/or a single fiber composition or, essentially, only(95% by weight or better) two materials, so that the final composite canbe recycled readily using conventional means. Although variousembodiments have been illustrated, this is for the purpose ofdescribing, but not limiting the invention. Various modifications whichwill become apparent to those skilled in the art, are within the scopeof this invention described in the attached claims.

1-11. (canceled)
 12. An energy absorbent laminate comprising: (a) a pairof composite layers containing a resin-impregnated glass fabric or mathaving a first toughness; (b) a core layer laminated between said pairof composite layers having a second toughness which is greater than saidfirst toughness, said core layer having a greater elongation at breakthan said first layer; said core layer comprising poorly wetted orweakly bonded high modulus fibers which help to at least distributeloads due to shear, cutting and impact forces exerted on said compositeby at least a portion of said core layer delaminating from said pair ofcomposite layers or by bunching during said delamination in response tosaid shear, cutting and impact forces.
 13. The composite of claim 12wherein said pair of composite layers comprise a woven or non-wovenfabric or mat made of high tensile modulus fiber, and said core layercomprises fibers, roving, yarn, woven fabric, non-woven fabric, tow orcombination thereof.
 14. The composite of claim 12 wherein said corelayer comprises low modulus polymeric filaments and filaments of atleast one high modulus reinforcing fiber selected from the groupcomprising: glass fiber, carbon fiber, boron fiber, aramid fiber or acombination thereof. 15-17. (canceled)
 18. A multi-layered compositelaminate, comprising: (a) a pair of resin-impregnated, fiber-containinglayers having a first flexural modulus and a first toughness; (b) afiber-containing core layer having a second, lower flexural modulus thansaid first flexural modulus and a second higher toughness than saidfirst toughness, said fiber-containing core layer sandwiched betweensaid pair of resin-impregnated, fiber-containing layers to form anintegral composite; (c) at least a portion of said fiber-containing corelayer configured to delaminate from said pair of resin-impregnated,fiber containing layers when subjected to shear, cutting or impactforces, or to bunch during said delamination, to provide improvedshearout resistance over a composite of approximately the same thicknessmade entirely from said resin-impregnated, fiber-containing layershaving a first flexural modulus.
 19. The composite of claim 18, whereinsaid pair of resin-impregnated, fiber-containing layers and saidfiber-containing core layer comprise substantially similar fibercompositions, resin compositions, or both, for improved recyclability.20. A multi-layered composite having improved energy absorbingproperties comprising: (a) a pair of resin-impregnated, fabric layersincluding high strength fibers; (b) a core layer for absorbing energydirected to said composite by externally applied forces, said core layercomprising polymeric fibers having greater toughness than said highstrength fibers, said core layer laminated between said pair ofresin-impregnated glass fabric layers so as to allow at least a portionof the core layer to delaminate from the resin-impregnated, fabriclayers, in response to said externally applied forces, or to bunchduring said delamination; said core layer laminated under heat,pressure, or both, to form an integral composite.
 21. The multi-layeredcomposite of claim 20, wherein said core layer is only partially meltedby said lamination.
 22. A ballistic and explosion-resistant panel,comprising: (a) a pair of resin impregnated, fiber-containing layers,having a first toughness; (b) an aramid fiber-containing core layerhaving a second, greater toughness than said first toughness, said corelayer sandwiched between said pair of resin impregnated,fiber-containing layers and joined to said resin impregnated,fiber-containing layers using an adhesive web to allow at least aportion of the core layer to delaminate from the resin impregnated,fiber-containing layers, in response to an externally applied force;said multi-layered composite having improved resistance to ballisticimpacts.
 23. The panel of claim 22 wherein said aramid fiber-containingcore layer comprises a woven or knit fabric.
 24. The panel of claim 22wherein said aramid fiber-containing core layer comprises a woven ornon-woven fabric having a basis weight of at least 300-6,000 g/m².
 25. Amulti-layered composite, comprising: (a) a pair of resin impregnated,fiber-containing layers; (b) a fiber-containing core layer sandwichedbetween said pair of resin impregnated, fiber-containing layers; saidcore layer comprising poorly wetted or weakly bonded high modulus fiberssuch that said core layer can absorb high amounts of energy by at leasta portion of said fibers delaminating from said pair of resinimpregnated, fiber-containing layers, or by bunching during saiddelamination.
 26. The multi-layered composite of claim 25, wherein thefiber-containing core layer has a second, greater toughness than saidfirst toughness.
 27. The multi-layered composite of claim 25, whereinthe fibers of said fiber-containing core layer comprise yarn, roving,two, woven fabric, non-woven fabric, or combinations thereof.
 28. Themulti-layered composite of claim 25, further comprising a polypropylenecopolymer wax to substantially eliminate air voids in the laminatestructure and render the composite translucent.
 29. The multi-layeredcomposite of claim 25, wherein the pair of resin impregnated,fiber-containing layers have a first toughness, and the fiber-containingcore layer has a second toughness, said first and second toughnessesbeing unequal.
 30. The multi-layered composite of claim 29, wherein saidfiber-containing core layer comprises aramid fibers.
 31. Themulti-layered composite of claim 25, wherein the fibers of said resinimpregnated, fiber-containing layers have the same composition as thefibers of said fiber-containing core layer, and wherein the fibers ofsaid fiber-containing core layer comprise a fabric having a lower basisweight as compared to that of the resin impregnated, fiber-containinglayers.
 32. The multi-layered composite of claim 25, wherein thefiber-containing core layer comprises aramid fibers, the core layerjoined to said resin impregnated, fiber-containing layers using adhesivewebs.
 33. The multi-layered composite of claim 25, wherein thefiber-containing core layer comprises aramid fibers and is joined tosaid resin impregnated, fiber-containing layers by heating and pressingthe layers together.
 34. The multi-layered composite laminate of claim18, wherein said fiber-containing core layer comprises poorly wetted orweakly bonded high modulus fibers.